1. Field of the Invention
This invention relates generally to a system and method for performing an anode exhaust gas bleed to remove nitrogen from the anode side of a fuel cell stack and, more particularly, to a system and method for performing an anode exhaust gas bleed to remove nitrogen from the anode side of a fuel cell stack in the event that a bleed manifold unit (BMU) fails.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The MEAs are permeable and thus allow nitrogen in the air from the cathode side of the stack to permeate therethrough and collect in the anode side of the stack, referred to in the industry as nitrogen cross-over. Even though the anode side pressure may be higher than the cathode side pressure, the cathode side partial pressures will cause air to permeate through the membrane. Nitrogen in the anode side of the fuel cell stack dilutes the hydrogen such that if the nitrogen concentration increases beyond a certain percentage, such as 50%, the fuel cell stack becomes unstable and may fail. It is known in the art to provide a bleed valve at the anode exhaust gas output of the fuel cell stack to remove nitrogen from the anode side of the stack.
An algorithm may be employed to provide an online estimation of the nitrogen concentration in the anode exhaust gas during stack operation to know when to trigger the anode exhaust gas bleed. The algorithm may track the nitrogen concentration over time in the anode side of the stack based on the permeation rate from the cathode side to the anode side, and the periodic bleeds of the anode exhaust gas. When the algorithm calculates an increase in the nitrogen concentration above a predetermined threshold, for example 10%, it may trigger the bleed. The bleed is typically performed for a duration that allows multiple stack anode volumes to be bled, thus reducing the nitrogen concentration below the threshold.
Some fuel cell systems employ anode flow shifting where the fuel cell stack is split into sub-stacks and the anode reactant gas flows through the split sub-stacks in alternating directions. In these types of designs, a bleed manifold unit (BMU) may be provided that includes valves for providing the anode exhaust gas bleed. Because water is present in the anode exhaust gas, it is likely that the BMU will have water remaining in it at system shut-down regardless of what measures are taken to remove the water. This water may freeze if the outside ambient temperature is low enough for a long enough period of time. On the next start-up, an anode exhaust gas bleed may be required before the BMU is thawed out enough, where ice may block the flow in the BMU. In certain fuel cell system designs, a continuous anode exhaust bleed is performed during the start-up sequence because the fuel cell stack is particularly sensitive to nitrogen collected during that time.
For a split stack system, the typical location to provide the anode exhaust gas bleed is at the end of the stack flow. Therefore, two bleed values are used to provide the anode bleed depending on the flow direction. Because a BMU is often provided to accommodate this form of bleeding it is typically referred to as the BMU bleed method. However, a center bleed also can be used that bleeds the anode exhaust from a drain valve in a line joining the two sub-stacks. The center bleed is typically less efficient that an end flow or BMU bleed because of the larger size of the drain valve.
The location for the bleed is one characteristic and the frequency and duration of the bleed is another characteristic. For a frozen stack, the bleed valve should be opened as much as possible to avoid any localized water build-up. This is referred to as a continuous bleed and can be an inefficient method of bleeding because hydrogen is also lost during the anode exhaust gas bleed. Thus, the system should return to a normal bleed schedule once the system is warmed up. The normal bleed method should be providing the bleeds as infrequently as possible to maximize system efficiency, while still maintaining good stack operation. In this mode, the bleed valves can be closed a significant percentage of the time during operation.